1. Field of the Invention
In general, the present invention relates to an infrared imaging apparatus and, more particularly, relates to correction of shading included in data of a picture taken by infrared imaging apparatus.
2. Description of the Related Art
Each substance radiates substance-temperature-dependent electromagnetic waves caused by motions of atoms or molecules on the surface of the substance unless the temperature of the substance is equal to the absolute zero. The maximum wavelength of electromagnetic waves radiated by many substances on the earth has a value in an infrared range. An infrared imaging apparatus is an apparatus for carrying out image processing by detection of infrared rays. In many cases, an infrared imaging apparatus is used in light reception equipment. One of most outstanding features of an infrared imaging apparatus is a characteristic that allows such equipment to be designed into small and light one. In general, such light-receiving equipment is referred to as a passive system. On the other hand, a system comprising a set of a transmission apparatus and a reception apparatus is known as an active system. Since an infrared passive system does not radiate an electromagnetic wave by using a transmission apparatus as a radar does, such a system has a characteristic of high concealability. For this reason, such a system has been developed for military purposes and has become the base of development of infrared technologies. At the present time, a number of application products centered at an image processing apparatus are available in the market as consumer products. The reception apparatus of the infrared system comprises a camera head for detecting infrared rays and converting the infrared rays into electrical signals, an A/D converter for converting an analog signal into digital data and an image-processing unit for processing the digital data representing an image in accordance with an application.
FIG. 32 is a diagram showing the general configuration of the camera head. As shown in the figure, the camera head 2 comprises an optical system 4 and an infrared detector 6. The optical system 4 comprises a lens 8 and a lens housing 10. The lens 8 condenses infrared rays. The lens housing 10 plays roles of supporting the lens 8 and preventing reflection of infrared rays by absorption of the infrared rays introduced to the inside of the lens housing 10. Such reflection is a cause of noise. The infrared detector 6 comprises a window 12, a cold shield 14, an infrared sensor 16, an inner shell 18 and an outer shell 20. The window 12 is a window for passing through infrared rays. The cold shield 14 plays a role of reducing the quantity of an unnecessary infrared ray hitting the infrared sensor 16. The infrared sensor 16 plays a role of outputting electrical signals with a level proportional to the intensity of incident infrared energy. The inner shell 18 and the outer shell 20 play a role of accommodating the infrared sensor 16.
FIG. 33 is a diagram showing a typical configuration of the camera head 2. The lens 8 shown in the figure comprises a plurality of lenses 8a to 8d. The lens 8a is made of Si while the lens 8b is made of ZnSe. On the other hand, the lens 8c is made of Ge while the lens 8d is made of Si. The window 12 is made of Ge and the cold shield 14 is a metallic plate. The infrared sensor 16 is made of semiconductor such as Hg1xe2x88x92xCdx, Te or Pb1xe2x88x92xSnxTe. The inner shell 18 and the outer shell 20 are each made of a metal such as kovar.
FIG. 34 is a diagram showing a typical implementation of the infrared detector 6 shown in FIG. 32. The infrared detector 6 is a vacuum thermal-insulating container having a dual structure comprising the inner shell 18 and the outer shell 20. On a portion of the outer shell 20, the window 12 is provided. On the inner shell 18 facing the window 12, the infrared sensor 16 is mounted. The inner shell 18 of the vacuum thermal-insulating container accommodates refrigerant such as liquid nitrogen. As an alternative, a cryostat 28 adopting a Joule-Thompson law operates at a predetermined temperature. The cold shield 14 is provided so as to enclose the infrared sensor 16. The cold shield 14 reduces the quantity of an unnecessary infrared ray entering the infrared sensor 16. Electrodes of the infrared sensor 16 and their conductor patterns are connected to each other by bonding wires 22 adopting a bonding technique. Infrared rays detected by the infrared sensor 16 is output to an external device as analog electrical signals appearing on lead pins 26 which are connected to semiconductor patterns by bonding wires 24. The analog electrical signals output from the lead pins 26 are each converted by the AD converter into digital data consisting of a predetermined number of bits. The digital data is supplied to the image-processing unit such as an apparatus for keeping track of an observation target or implementing medical treatment. In the image-processing unit, the digital data is subjected to various kinds of image processing.
FIG. 35 is an explanatory diagram used for describing a role played by the cold shield 14. As described above, the cold shield 14 is provided so as to enclose the infrared sensor 16. The inner surface of the cold shield 14 is coated with a black coating material. Baffles 30 are provided on the walls of the inner surface. The baffles 30 each reduce the quantity of an unnecessary infrared ray entering the infrared sensor 16. The cold shield 14 is designed so that infrared rays in a range denoted by reference numeral 32 are condensed by the lens 8 at a position A on the surface of the infrared sensor 16. In general, about a photographed picture output by the camera head 2 infrared rays are incident on the surface of the infrared sensor 16 not in a uniform irradiance distribution even if the picture is taken as a result of photographing a scene or an image-taking object having a uniform distribution of radiation intensities. Instead, a signal output by the infrared sensor 16 shows a quadratic-function distribution with respect to the position of a field of view. This quadratic-function distribution is a phenomenon known as shading. If the shading phenomenon becomes too excessive, an accurate picture of the scene or the objects of image-taking cannot be taken, and the objective of the image-taking cannot be achieved in some cases. In order to reproduce accurate picture information of a scene or another image-taking object by using an image-taking unit, it is necessary to adopt a shading correction method capable of effectively removing only shading components from a signal generated by the infrared sensor 16.
FIG. 36 is an explanatory diagram used for describing the aforementioned shading phenomenon. The shading phenomenon occurring in an infrared imaging apparatus includes two components, namely, a shading component caused by an optical system and a shading component caused by a housing comprising the lens housing 10, the inner shell 18 and the outer shell 20. The shading component caused by the optical system is a shading component due to irradiance distribution which is developed on the surface of the infrared sensor 16 when an image is created by scene components 40 passing through an effective aperture of the optical system. On the other hand, the shading component caused by the housing parts containing the lens housing 10, the inner shell 18 and the outer shell 20 is a housing components 42 incident to the infrared sensor 16. The infrared rays are radiated by the lens housing 10, the inner shell 18 and the outer shell 20 themselves, which constitute the housing as described above. The shading component caused by the housing parts is peculiar to an infrared imaging apparatus and does not exist in a visible image-taking apparatus. The most important problem raised in the shading correction method adopted by the infrared imaging apparatus is how to effectively correct a housing component. As a remark, it should be noted that a housing component is different from a housing-shading component as follows. The housing component is a component of a ray incident to a sensor device. The housing component is originated from the housing parts. On the other hand, the housing-shading component is used in comparison with the shading component caused by the optical system. The housing-shading component is a shading component caused by a housing component. In the following description, a housing-shading component is also referred to as a shading component caused by the housing. As a shading correction method focusing on a shading component caused by the housing parts, the infrared imaging apparatus adopts the following commonly-known technologies.
In accordance with a technology disclosed in Japanese Patent Publication No. Hei 7-32467 and referred to hereafter as prior art 1, the temperature of a lens housing employed in an optical system is measured by using a temperature sensor. A housing component is then computed from the measured temperature and a cubic angular table representing the lens housing""s view seen from detector elements. The cubic angular table is stored in a memory. A shading component caused by the housing parts is corrected by subtracting a result of the computation from picture data. A shading component caused by the optical system is corrected by multiplying a result of the subtraction by irradiance distribution data stored in the memory.
In accordance with a technology disclosed in Patent No. 273196 and referred to hereafter as prior art 2, a picture signal of a plurality of lines which is generated by a linear array detector is stored. The picture signal is integrated and averaged for each element of the linear array detector and, by subjecting the averaged picture signal to a low-pass filtering, a shading component is extracted and removed from the original picture signal.
In accordance with a technology disclosed in Japanese Patent Laid-open No. Hei 5-292403 and referred to hereafter as prior art 3, an adjusted focus of an optical system is temporarily shifted to make a picture blurring. By this, even in the case where the radiation intensity of a scene is not distributed uniformly, uniform infrared rays are incident on detector elements and outputs of the detector elements are used as correction data.
In accordance with another technology disclosed in Japanese Patent Laid-open No. Hei 8-223484 and referred to hereafter as prior art 4, while a view axis of an image-taking unit is forcibly driven into a scanning movement, signals output by each infrared detector during a normal image-taking operation are averaged and smoothed. The averaged and smoothed signal is used for continuous shading correction in the normal image-taking operation. This technology is characterized that view axis scanning and signal smoothing are combined to exhibit the same effect as prior art 3. That is to say, by realizing a state equivalent to a case in which a uniform infrared ray is radiated to all detector elements on the average along the time axis even if the distribution of the radiation intensity of the scene is non-uniform, corrected data is obtained. To put it concretely, a deviation of each pixel from an average of all pixels is added to or subtracted from picture data of an object of correction. The deviation is a difference from an average value calculated for all pixels in calibration data created from smoothed data.
However, the prior arts have the following problems.
In accordance with prior art 1, a housing component is computed by using a measured temperature of a lens housing 10 and a cubic angular table representing the lens housing""s view seen from detector elements as described above. In the case of the housing component caused by infrareds incident to detector elements from the lens housing 10, there are a number of components that can be calculated so that relatively accurate correction is possible. In the case of the housing component caused by an incident infrared ray originated from infrared detectors/containers such as the window 12, the cold shield 14, the inner shell 18 and the outer shell 20, on the other hand, there are a number of components that are difficult to calculate so that accurate correction by calculation is impossible. A housing component caused by an incident infrared ray from an infrared detector container normally has a level which is not negligible when compared with a housing component originated from infrared rays coming from the lens housing 10. Thus, with prior art 1, the shading component caused by the housing cannot be corrected with a high degree of accuracy and accurate shading correction is thus impossible. In addition, in the case of an infrared imaging apparatus mounted on a movable structure of a gimbal, which is required to have a small size and a fast response such as a missile seeker, for example, prior art 1 also has a problem of deterioration of movable-unit driving performance due to wires added for temperature measurements.
In accordance with prior art 2, an attempt is made to extract a shading component directly from an output of each detecting elements during an image-taking process without the need for a temperature measurement in order to solve the problems encountered in prior art 1. However, the low-pass filtering raises a problem of impossibility to extract an accurate shading component. That is to say, an incident component coming from a scene generally contains a variety of spatial frequency components mixed with each other. It is thus difficult to discriminate a shading component and a scene component from each other by using spatial frequencies. As a result, in actuality, it is difficult to extract only the shading component.
Prior art 3 raises a problem of temporary suspension of the objective to use the image-taking unit by an operation to shift the focus. In addition, prior art 3 also raises a problem of impossibility to implement accurate shading correction due to lost conformity of correction data. The non-conformity of correction data is caused by changes in housing temperature and scene temperature which occur since acquisition of the correction data.
While shading correction is possible, prior art 4 raises a problem of an altered scene picture which is resulted in as follows. If a scene component includes a structural distribution such as a ridge line resembling a brow of a hill, the scene component remains in smoothed data, making the data non-uniform. The non-uniform data is reflected in picture data obtained from correction processing.
It is thus an object of the present invention to provide an infrared imaging apparatus capable of reproducing an accurate scene picture by carrying out good shading correction even if the scene temperature and/or the housing temperature change.
In accordance with an aspect of the present invention, there is provided an infrared imaging apparatus capable of carrying out shading correction of picture data obtained as a result of an image-taking process using a camera head comprising an optical system, a plurality of detector elements and a container for accommodating said detector elements, said infrared imaging apparatus characterized by including a first correction unit for creating corrected-sensitivity picture data by correction of shading components caused by the optical system to produce uniform scene components included in the picture data obtained as a result of an image-taking process of a uniform scene; a storage unit for storing a housing response profile for correcting a housing shading component caused by infrared rays radiated by the optical system and the container for each of the detector elements; and a second correction unit for creating corrected-housing-shading picture data by correction of housing-shading components based on the corrected-sensitivity picture data and the housing response profile for each of the detector elements.
Preferably, the second correction unit may correct a housing-shading component by executing the steps of assuming that, for each of the detector elements, the corrected-sensitivity picture data of the detector element is a sum of a housing component of the detector element and a second constant representing a scene component where the housing component is a product of a first constant and the housing response profile for the detector element. Then finding the first constant""s value that minimizes a total obtained by summing square of a difference of the sum from the corrected-sensitivity picture data related to the detector elements; and subtracting a product of the housing response profile of the detector element and the first constant from the corrected-sensitivity picture data of the particular detector element for each of the detector elements.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best understood from a careful study of the following description and appended claims with reference to attached drawings showing some preferred embodiments of the invention.